Django Rest Framework API Client Custom Header

Your initial solution is almost correct; you are just missing the "X" portion of your header field name:

self.client.credentials(HTTP_X_BULK_OPERATION='true')

This worked for me with other arbitrary header keys.

Or you can just pass it as a kwarg of request.
It is important to add HTTP_ prefix to your header (HTTP_MyHeader, instead of MyHeader):

response = self.client.patch(
    '/api/v1/db_items/?active=True',
    json.dumps(data),
    content_type='application/json',
    'HTTP_X_BULK_OPERATION'='true'
)

The answer is extremely poorly documented, but it seems django does its own parsing of the headers passed in. I successfully did this by changing my code to be

response = self.client.patch(
    "/api/v1/db_items/?active=True",
    json.dumps(data),
    content_type="application/json",
    HTTP_X_BULK_OPERATION="true",
)

Note the HTTP_ prefix. This is recognized by the Django client and translated to a HTTP header with the key X-BULK-OPERATION and the value "true"